Microelectromechanical systems (MEMS) technology has achieved wide popularity in recent years, as it provides a way to make very small mechanical structures and integrate these structures with electrical devices on a single substrate using conventional batch semiconductor processing techniques. One common application of MEMS is the design and manufacture of sensor devices. The electromechanical portion of the device provides the sensing capability, while the electronic portion processes the information obtained by the electromechanical portion. One example of a MEMS sensor is a MEMS gyroscope sensor.
Alternatively referred to as a “gyroscope”, “gyrometer,” “angular rate sensor,” or “yaw rate sensor,” a gyroscope sensor senses angular speed around one or more axes. One type of MEMS gyroscope sensor uses a vibrating element to sense angular rate through the detection of a Coriolis force, or acceleration. The vibrating element is put into oscillatory motion in the X-axis (drive plane), which is parallel to the substrate. Once the vibrating element is put in motion, it is capable of detecting angular rates induced by the substrate being rotated about the Z-axis. The Coriolis acceleration occurs in the Y-axis (sense plane), which is perpendicular to both the X-axis and the Z-axis. The Coriolis acceleration produces a motion having an amplitude that is proportional to the angular rotation rate of the substrate.
In electrical circuits, parasitic capacitance is the unavoidable and typically unwanted capacitance that exists between the parts of an electronic component or circuit due, in part, to their proximity to one another. In addition, all actual circuit elements such as, inductors, diodes, and transistors have internal parasitic capacitance, which can cause their behavior to depart from that of “ideal” circuit elements. Parasitic capacitance can also exist between closely spaced conductors, such as wires or printed circuit board traces. The parasitic capacitance may be inherent in a MEMS sensor or the associated packaging and bonding arrangement, so that the parasitic capacitance values could change not only for different sensor implementations, but the parasitic capacitance values could vary from unit-to-unit in production.
A MEMS gyroscope sensor has parasitic capacitance between the drive nodes and the sense nodes of the device which produces an error in the signals corresponding to sensor position. Parasitic capacitances between the drive nodes and the sense nodes are particularly troublesome, since the parasitic capacitances produce currents that are in quadrature with the desired sensor position signal. Thus, an error is created in the signal determined at the sense nodes so that the position of the vibrating element is determined in error.
Some approaches involve combining capacitors directly in parallel through a switch in series with each capacitor to create a one-port capacitive network with variable capacitance. Unfortunately, because of the parasitic capacitance inherent in the switches, as well as the minimum physical size of the capacitors for a MEMS sensor implementation, the minimum capacitance of the switched elements cannot reach zero. Moreover, in such a one-port capacitive network, the minimum achievable capacitance can increase as the number of switched capacitors increases. Limitations in the minimum physical size of the capacitors and an increase in the minimum achievable capacitance as the number of switched elements increases are highly undesirable in some MEMS gyroscope sensors where parasitic capacitance values can be in a range of approximately one half to fifty femtofarads.